This invention relates to phenolic compounds and a heat-sensitive recording material containing the same as a color developer.
As already known from Japanese Patent Publication No. Sho 43-4160/1968, No. Sho 45-14039/1970, Japanese Patent Application Laid-Open No. Sho 48-27736/1973, etc., heat-sensitive recording materials (hereinafter abbreviated merely to recording materials) consist of a heat-sensitive recording layer composed mainly of a colorless or light-colored basic color-forming substance (dye) and an acidic color developing substance (color developer) and a substrate supporting the layer, and when the dye and the color developer are subjected to fusion by heating, a recorded image is obtained by forming a deep, developed color phase. Such heat-sensitive recording materials have been broadly applied mainly to instrumentation recording, terminal recording of information and communication apparatus, output recording of computer, etc.
However, the advance of heat-sensitive recording apparatus and the extension of the application field of the recording materials have been becoming notable, and along with this, a higher level of the qualities of the recording materials have been required. As particularly desired qualities, the whiteness of the recording materials, the stability of records, and the heat response properties of the recording materials may be exemplified.
The whiteness of the recording materials is a specific feature which makes clearer the resulting recorded images which in turn resort directly to vision; hence the whiteness should be said to be an important quality of the materials. To reduce the whiteness of the recording materials has been usually called "fogging" and it is often observed when the color developer used has a particularly low melting point or is simewhat water-soluble. During the production process of the recording materials or during their storage, a color development already somewhat occurs over the surface thereof. Particularly in the case of bisphenol A having often been used as a color developer, it has been known that fogging is caused by its water-solubility. In order to prevent this fogging, a method of adding a slightly desensitizable substance has occasionally been employed, but the method is said to be undesirable since it reduces the intrinsic color-forming properties of the recording materials.
The stability of records represents how long the records do not lose their images in an environment wherein the records are usually preserved. As to the cause for which the records are damaged, many ones have already been known such as (1) reduction in the concentration of developed color phase due to vaporization of color developers, (2) reduction in the concentration of developed color phase due to photochemical deterioration, (3) densensitizing function of developed color due to environmental substances, (4) extinction of developed color phase due to phase separation of developed color phase or bleeding of color developer from developed color phase, etc.
The heat response properties represent in what short time or with what minute energy for heating the development of the recorded images is carried out. This specific feature has usually been supplemented by adding a third substance (sensitizer).
Generally bisphenol A (2,2-di(4-hydroxyphenyl)propane) has often been used as a color developer. Bisphenol A has a deep color-developing function, but it is practically considerably unsatisfactory in respect of the whiteness of the recording materials, the stability of records or the heat response properties of the recording materials.
On the other hand, 1,1-di(4-hydroxyphenyl)cyclohexane has been known to provide a recording material which is inferior in the heat response properties of recording materials but particularly superior in the stability of records. This is presumed to be caused by either a fact that 1,1-di(4-hydroxyphenyl)cyclohexane forms a developed color phase having an extremely high viscosity with dyes, or a fact that the compound has a specifically strong interaction with dyes.
The causes for which the stability of records is damaged are as described above; however, particularly when it is required to preserve the records for a long time, the phase separation of the color developed phase may be a most important problem. The developed color phase consists of a thermodynamically unstable non-crystalline phase, and there is a tendency that at least one of the color developer and the dye contained therein causes much or less phase separation from the non-crystalline phase in the form of an independent crystalline phase, respectively. When the color developed phase is extinct, the color-developed concentration becomes nil; hence the records are lost. The extinction of the color-developed phase due to the phase separation is easily confirmed since the phase is temporarily regenerated by reheating. In general, the resistance to the tendency of the phase separation consists in the viscosity of the color-developed phase. The higher the viscosity of the color-developed phase, i.e. the higher the glass transition point of the color-developed phase, the slower the tendency of the phase separation.